Thread: How to find the last value in an array

Hi experts,
I am new to C, i need to insert an element in the middle of an array,for that i thought a logic that after finding the last value in an array i can shift the value and after that i can insert the new value for this i have to find the last value in an arry,how can i find the last value in an array.

example

int arr[30]



in this i have to insert the element after the 4 location,for that i thought to find the last value in an array

ex the last value in the arr[15].


thanks and regards,
kanimozhi.m

So the array is obviously not full, right? If the array is full the last element is obviously arr[29]. So I assume its not full.

Since your using a primitive type ("int"), there is no "NULL" value. Therefore you have to decide on some "delimiter" to use. So if your array should only hold positive numbers, then the delimiter should be a negative number, say -1. So you would create an array and initialize all values to -1, then use the array.

If you cant modify the array, then you need to keep a counter to say how many values are actually being used.

If neither of these are possible, then it cannot be done. For example, if the array can store any value, then you cannot have a delimiter. So the only way is to use a counter. If you cant use a counter, then its not possible.

So let us know which case your situation applies to.

Code:

#include <iostream>

int
main (int argc, char * argv[])
{
  int x[] = {1,2,3,4,5};

  std::cout << *((*(&x+1))-1) << std::endl;
}

last element printed, awesome.

Originally Posted by edoceo

Code:

#include <iostream>

int
main (int argc, char * argv[])
{
  int x[] = {1,2,3,4,5};

  std::cout << *((*(&x+1))-1) << std::endl;
}

last element printed, awesome.

have an extra set of parens there, no big deal though

Code:

   std::cout << "6: " << *(*(&x+1)-1) << std::endl;

I know this works, but real tricky code. Can you help explain how this works?

Originally Posted by anon

I hope he's just messing with the OP's brain.

But basically the idea is to move a pointer by sizeof(x) (where x is int[5]) and then decrement the pointer by sizeof(int).

Not entirely sure if there's also some undefined behavior involved.

Honestly that messed with my brain, but I see the picture through the murkiness of pointer arithmetic.The jump to 20bytes got me. Ive testing with adding up to 3 more int's and that still gets the last element, still cool how that works.

Originally Posted by anon

If you want to give people a chance to understand what it is doing, you might also try

Code:

#include <iostream>

int main()
{
  int x[] = {1,2,3,4,5};

  std::cout << x[sizeof(x)/sizeof(*x) - 1] << std::endl;
}

And if you want to make it even cleaner, and at the same time let the compiler ensure that you are in fact dealing with an array (and not an array that has decayed to a pointer and thus lost track of the array's size):

Code:

#include <iostream>

template <class T, unsigned N>
unsigned array_size(const T (&)[N])
{
    return N;
}

int main()
{
      int x[] = {1,2,3,4,5};
      std::cout << x[array_size(x) - 1] << std::endl;
}

I really had to read up on that as I was stuck on the array decay to pointer and the unique way sizeof() functions with that. This thread cleared that up for me.

Newbie question about 'sizeof' and arrays

Ok I'm done here...

sizeof - Wikipedia, the free encyclopedia

"sizeof can only be applied to "completely" defined types. With arrays, this means that the dimensions of the array must be present in its declaration, and that the type of the elements must be completely defined."